The Sign in Structuralism

I keep reading and re-reading this essay on the The Arbitrary Nature of the Sign and still cannot fully grasp what these concepts mean and how they apply to art. I guess what really trips me up is that Saussure is trying to say that we understand concepts because we can negate all the concepts that do not apply to the one in our mind, or "... any sign is to be what all other signs are not." Thus, signs are defined by their opposites and our ability to keep one sign separate from another.
I guess that this is sort of present throughout art history. If we look at Baroque art we can see that each signifier is clearly separate and we understand that this is a painting of a man, or a fruit basket, etc.
The book applies this to Structuralism and uses Picasso's Bull Head as an example. The book says that this example shows that any sign can mean anything as long as it follows the rules of linguistics, but does that not go against what the structuralists believe? If there is an underlying syntax that governs how or world works, a law of binary, does that not break those laws and show that two can become one? Bicycle parts can became an animal?